The immune response system contributes to the body’s defence against infection, toxic or allergenic substances and is concerned with the recognition of tumour cells. In responding to a challenge the immune system is able to distinguish the body’s own cells and components(self) via the major histocompatability complex (HLA-DR) class 1 from cells that are foreign (non-self). The abnormalities of the immune response is demonstrated in the immunodeficiency diseases (congenital and acquired), the hypersensitivity reactions that may be involved in producing autoimmune diseases and the switching-off of T cell function by cancer cells. The genetic regulation of the immune system have major implications in clinical medicine as to the understanding of autoimmune disease and the idiotypic network that militates against autoimmune response and excessive immune responses. The relationship between immune function and tumour cells is highly complex but crucial to the understanding of both tumour rejection and progression mechanisms The improved knowledge of the immune system has expanded the role of immunotherapy and vaccine therapy in oncology. The article reviewed the essential immune mechanisms in health and disease, and the clinical implications.